The present invention is directed, in general, to surface acoustic wave circuits and, more specifically, to a hermetic package for a surface acoustic wave circuit and method of manufacturing such hermetic package therefor.
Electronic signal processing by means of surface acoustic wave (SAW) devices has been widely adopted by the electronics industry. Such SAW devices can be designed to operate as analog electrical filters that operate at over a wide range of frequencies and have several advantages over conventional technologies. One such advantage is that they can be designed to provide complex signal processing in a single unit. Saw devices also benefit from the ability to be mass produced using semiconductor microfabrication techniques which produces highly uniform devices at a substantially reduced cost. SAW devices can be easily integrated into many digital communications systems and designed to operate in high harmonic modes in the gigahertz (GHz)frequency range.
The response characteristics of a particular SAW device are governed by several factors. One is the geometry of conductors laid out on the SAW resonator""s piezoelectric substrate. A typical geometry for a SAW resonator includes first and second SAW finger sets. Portions of the finger sets are interdigitated in a central region of the SAW resonator and are employed to generate or attenuate acoustic waves. Additional non-interdigitated finger sets lie outside of the central region and serve to reflect acoustic waves back into the central region. Proper operation and containment of the acoustic waves require precise construction of both the central and outlying regions.
The interdigitated finger sets act as input and output signal ports when an AC voltage is applied to the signal input portion of the metal lines. Application of an appropriate input electrical AC signal provides the stimulus to create an acoustic wave that may typically be a Rayleigh wave with motion confined to about one acoustic wavelength under the free surface of the piezoelectric substrate. Alternatively, the acoustic excitation may be a xe2x80x9cleaky wave,xe2x80x9d which also finds application in modern radio frequency devices. This wave is propagates to the receiver portion. The fingers corresponding to the signal receiving portion draw energy from the acoustic wave in the lattice and convert it into a filtered electrical signal.
However, effective operation at high frequencies and general reduction in device size require a SAW resonator with smaller, more closely spaced finger sets. An undesirable effect of these small geometries is that the metal lines become subject to failure. One particularly troublesome mechanism of failure resulting from formation of large voltage gradients in the piezoelectric substrate. Such voltage gradients may develop as a result of thermal gradients developed during heating cycles in the manufacturing process, given the inherent pyroelectric nature of the piezoelectric substrate. When the gradient exceeds a threshold, arcing may occur, damaging or destroying the ability of the interdigitated metal lines to transmit and detect the surface acoustic wave.
Accordingly, what is needed in the art is a surface acoustic wave device and a method of manufacturing a surface acoustic wave device that reduces or eliminates the damage to the device resulting from the presence of voltage gradients in the piezoelectric substrate.
To address the above-discussed deficiencies of the prior art, the present invention provides a hermetic package for a pyroelectric-sensitive electronic device and methods of manufacturing one or more of such packages. In one embodiment, the package includes: (1) a device substrate having: (1a) an active region containing an electrically conductive pattern that constitutes at least a portion of the device and (1b) a bonding region surrounding the active region, (2) a non-porous mounting substrate having a bonding region thereon and (3) a nonmetallic hermetic sealing adhesive, located between the bonding region of the device substrate and the bonding region of the mounting substrate, that cures at a temperature substantially below a pyroelectric sensitive temperature of the device, the active region proximate a void between the device substrate and the mounting substrate.
The present invention therefore introduces the broad concept of forming a hermetic seal for a pyroelectric-sensitive device of a nonmetallic sealing adhesive. Curing the adhesive involves relatively low temperatures, thereby avoiding any substantial pyroelectric effects that might harm the device.
In one embodiment of the present invention, the active region is sunken into the device substrate relative to the bonding region thereof to accommodate at least a portion of the void. In an alternative embodiment, a central region of the mounting substrate is sunken into the mounting substrate relative to the bonding region thereof to accommodate at least a portion of the void. In another alternative embodiment, both the device and mounting substrates have sunken portions that together accommodate the void.
In one embodiment of the present invention, the adhesive has a cure temperature of at most 150xc2x0 Fahrenheit. Of course, other cure temperatures and ranges are within the broad scope of the present invention.
In one embodiment of the present invention, the device substrate comprises at least one selected from the group consisting of: (1) bismuth germanium oxide, (2) gallium arsenide, (3) lithium borate, (4) lithium niobate, (5) lithium tantalate, (6) langasite, (7) lead zirconium tantalate and (8) quartz. Those skilled in the art will understand, however, that other materials may be suitable for use as a device substrate, depending upon the particular application involved.
In one embodiment of the present invention, the mounting substrate comprises at least one selected from the group consisting of: (1) ceramic, (2) silicon and (3) glass. Those skilled in the art will understand, however, that other materials may be suitable for use as a mounting substrate, depending upon the particular application involved.
In one embodiment of the present invention, the adhesive comprises at least one selected from the group consisting of: (1) acrylate coated with silicon nitride, (2) acrylate coated with silicon carbide, (3) acrylate coated with silicon oxide, (4) acrylate coated with aluminum nitride, (5) acrylate coated with aluminum oxide, (6) benzocyclobutene coated with silicon nitride, (7) benzocyclobutene coated with silicon carbide, (8) benzocyclobutene coated with silicon oxide, (9) benzocyclobutene coated with aluminum nitride, (10) benzocyclobutene coated with aluminum oxide and (11) epoxy resin. Those skilled in the art will understand, however, that other materials may be suitable for use as an adhesive, depending upon the particular application involved.
In one embodiment of the present invention, the device is selected from the group consisting of: (1) a surface acoustic wave device, (2) a micro-electromechanical system device, (3) a mirror device and (4) a piezoelectric device. Those skilled in the art will understand, however, that other devices are within the broad scope of the present invention.
In one embodiment of the present invention, the device substrate and the mounting substrate have coextensive footprints. Noncoextensive footprints are within the broad scope of the present invention.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.